Why is it significant that the poem is set in the winter, on the darkest evening of the year?

There are several elements in "Stopping by Woods on a Snowy Evening" by Robert Frost that demonstrate the feelings and mindset of the speaker. The speaker is alone on his journey, as demonstrated by the owner of the woods being unaware of his presence and by the fact that he is stopping "without a farmhouse near." The speaker has stopped to "watch the woods fill up with snow"; he is admiring the beauty of the woods and falling snow around him. Perhaps he is thinking of home, a warm and safe sanctuary from the cold and snow, or he could be thinking of his destination and the journey he still has ahead of him. The speaker may be getting weary, and the quiet and darkness of the woods represent the desire for rest. The speaker says, "The woods are lovely, dark and deep," like thoughts of a sweet slumber are calling to him. The woods are quiet but for "easy wind and downy flake," a peaceful combination of sounds that could easily lull him to sleep. Alas, the speaker cannot stop to rest because he still has responsibilities to fulfill:

The woods are lovely, dark and deep,But I have promises to keep,And miles to go before I sleep,And miles to go before I sleep.

The setting of the poem paints a picture of the obligations the speaker has weighing down on him. The coldness of the winter evening, the quiet and isolation of being alone in the woods, and the darkness of the night all help to illustrate the things he must get done before he can rest. He has promises that must be fulfilled and many miles to travel before he can reach the light and warmth of his destination and finally sleep.
https://www.poetryfoundation.org/poems/42891/stopping-by-woods-on-a-snowy-evening